Suggestions for exercise on off SL 5x5 days?

I'm no expert. However, I'm currently doing this program too. My experience is that the StrongLifts 5 X 5 program is a full body workout especially centered around squats. I would not recommend doing additional weight training especially legs, on off days because you want your muscles to have plenty of time to recover. I generally do some cardio on my off days but that's my personal choice. If you are looking to add accessory exercises, the app has a list of recommended ones that go with each workout, but the 3 main exercises should be enough, especially for a beginner.